1. Field of the Invention
The invention relates to a method and an apparatus for shaft sealing and for cooling on the exhaust-gas side of a thermal turbomachine, in particular an axial-flow gas turbine.
2. Discussion of Background
It is known that thermal turbomachines, in particular axial-flow gas turbines, essentially consist of the bladed rotor and the blade carrier, which is equipped with guide blades and hung in the turbine casing. Adjoining the turbine casing is the exhaust-gas casing, which in modern machines is flanged to the turbine casing and essentially consists of a hub-side annular inner part and an annular outer part which define the exhaust-gas diffuser. The inner part and the outer part are connected to one another by a plurality of radial flow ribs arranged uniformly over the periphery. The outlet-side bearing arrangement of the turbine rotor is disposed in the hollow space inside the inner part, that is, inside the diffuser construction itself.
Shaft seals (labyrinth seals, gland) are present for the noncontact sealing of the leadthroughs of the rotor through the exhaust-gas casing and for reducing the leakage to a suitable proportion.
In order to prevent hot exhaust gases from being able to penetrate into the bearing space, compressor air has hitherto been extracted from a certain stage, directed via a separate line to the exhaust-gas casing and fed as barrier air directly into the gland on the exhaust-gas side. A portion of the air escapes through the seal into the bearing space, the rest flows along the shaft disk into the hot-gas duct.
If a compressor having one or more variable guide blades is used in a gas turbine and if these guide blades are closed by a certain amount in the partial-load range, this results in a lower pressure at the extraction point of the barrier air relative to the pressure during full-load operation. Therefore, so that there is sufficient barrier-air pressure in each operating state, either air has to be extracted at a high stage in which there is always sufficient pressure or a changeover has to be made between different stages.
The extraction of the air at a high stage has the disadvantage that highly compressed air is "consumed" at full load without power output, which has an adverse effect on the efficiency of the gas turbine. On the other hand, if a changeover is made between different stages, more extraction points at the compressor and changeover valves are necessary, so that the costs increase.
If cooling air has to be introduced through the exhaust-gas-side shaft end into the rotor, the rotor cooling air is also extracted from a certain compressor stage in addition to the barrier air and is fed via a special pipeline into the rotor. The transition of pipeline/rotor is here sealed off with labyrinth seals. The labyrinth leakage air passes into the surroundings of the bearing and leads to heating-up of the bearing space. This is undesirable, since the bearing temperature is limited because of the devices present, the bearing oil and the possibility of an inspection.
Apart from the leakage of barrier air and rotor cooling air, the bearing space is also heated up by the heat flow from the exhaust-gas stream through the insulation or the supporting structure. In most machines, the bearing space is cooled by natural convection. It is also known to cool the bearing space by cooling air which enters through openings in the exhaust-gas diffuser and leaves through the gap between lining and rib of the exhaust-gas casing. In this solution, the supporting structure of the exhaust-gas casing has no uniform temperature at the periphery, which disadvantageously leads to thermal stressing occurring and/or to the bearing no longer being concentric.